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Synthesis of carbon coated silica nanowires
Materials Chemistry and Physics 124 (2010) 88–91
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis of carbon coated silica nanowires Jiangtao Zhu, Fung-luen Kwong, Ming Lei, Dickon H.L. Ng ∗ Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, China
a r t i c l e
i n f o
Article history: Received 14 December 2009 Received in revised form 28 April 2010 Accepted 23 May 2010 Keywords: Nanostructures Chemical vapor deposition Electron microscopy Microstructure
a b s t r a c t Carbon coated silica nanowire hybrid structures were directly synthesized by a chemical vapor deposition method using ethanol as the precursor. The morphology and structural properties of the nanostructures were investigated. It was found that, in the sample prepared at a relative lower temperature (1185 ◦ C), the carbon shell was composed of orderly graphitic layers, whereas the carbon shell on those fabricated at higher temperature (1236 ◦ C) was amorphous. Moreover, the thickness of the amorphous carbon shell increased with increasing reaction temperature and reaction time. © 2010 Elsevier B.V. All rights reserved.
1. Introduction One-dimensional (1D) nanostructures such as nanowires, nanotubes and nanobelts have unique properties and promising applications in various fields [1–3]. However, due to their large aspect ratios, the surface features on 1D nanostructures are often found unstable and reactive to surrounding materials [4]. One of the remedies to overcome these problems is to modify the surfaces of these nanostructures. Carbon is one of the most popular materials for this purpose owing to its superior physical and chemical properties [5,6]. Amorphous carbon coated nanowires, in particular, were found to have significant enhanced field emission properties [7–11]. Currently, methods such as DC magnetron sputtering [7], RF induction heating chemical vapor deposition (RFCVD) under hydrogen flow [8], hydrogen arc discharge [9], and the one-step thermal process [10] have been employed to fabricate carbon coated nanostructures. In this work, we have developed a facile route to prepare graphitic carbon coated silica nanowires (NWs) using ethanol as a precursor at ambient pressure. The route had rather high productivity without the necessity of the strict processing condition (e.g. high vacuum) and the usage of hydrogen gas. In the product, a graphitic carbon cylindrical shell was formed around the silica NW core and a typical core–shell structure was produced. The presence of the graphite shell would increase the thermal stability of the silica NW. We had systematically studied the effects of different growing conditions on the thickness and crystalline
∗ Corresponding author at: Department of Physics, The Chinese University of Hong Kong, Room 108, Science Center North Block, Shatin, Hong Kong, China. Tel.: +852 2609 6392; fax: +852 2603 5204. E-mail address: [email protected] (D.H.L. Ng). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.05.071
structures of these silica NWs. A mechanism in the formation of this carbon coated nanostructure is proposed. 2. Experimental Thin layer of Au was coated onto the 1 cm2 Si square substrates (Wafer World, Inc. Test Grade) by ion sputtering a sputter coater (Polaron SC502, Fisons) before they were placed near the center of a horizontal tube furnace. The system was pumped down to 10−2 Torr and was flushed with Argon repeatedly. After returning to ambient pressure and heating up to the temperatures between 1000 ◦ C and 1300 ◦ C, Ar gas was allowed to enter the system at a rate of 40 ml min−1 after passing through an ethanol (99.9%, Merck) bath at 50 ◦ C. The reaction lasted for 30 min, and the samples were collected. A thermal profile of the furnace was measured to determine the exact temperature of each substrate during annealing. The morphologies of the products on the substrates were examined by scanning electron microscopy (SEM, LEO 1450VP). The structures of the products were analyzed by transmission electron microscopy (TEM, Philips CM120) and high resolution TEM (HRTEM, Tecnai 20ST). These microscopes were equipped with X-ray energy dispersive spectroscopy (EDS). Raman measurement was performed on this product by a micro-Raman spectroscope (Renishaw U1000B) utilizing a 514.5 nm solid-state laser as excitation source.
3. Results and discussion On the surface of the Si substrate prepared at 1185 ◦ C for 30 min, a layer of black substance was observed. The SEM revealed that the surface was covered with nanowires (NWs) having an average diameter of ∼500 nm and length of ∼20 m (Fig. 1a). The layer was scratched, and debris was collected and examined by TEM. It was found that these NWs had a core–shell structure (Fig. 1b). The diameter of the core (region A) varied (from 190 nm to 390 nm) along its long axis, whereas the thickness of the shell (region B) was rather uniform (∼80 nm). The corresponding EDS spectra (Fig. 1c) showed that the core (top curve) contained 37 at% Si, 53 at% O and 10 at% C, while the shell (bottom curve) contained 97 at% C, 2 at% O and
J. Zhu et al. / Materials Chemistry and Physics 124 (2010) 88–91
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Fig. 1. (a) SEM image of the sample prepared at 1185 ◦ C for 30 min; (b) mid segment of a typical core–shell wire; (c) EDS spectra obtained from the core region A and the shell region B in (b); (d) the SAED pattern of the region B, (e) dark field image of (b); and (f) HRTEM image of the edge of the wire (left was the surface of the graphite shell).
only 1 at% Si. It was evident that the core contained mainly silicon oxide, and was covered with a carbon shell. A selected area electron diffraction (SAED) pattern was obtained from region B (Fig. 1d). The two distinguish elongated spots in the SAED pattern indicated that the shell was composed of layered structure with preferred orientation, which was similar to the graphitic layer structure obtained in other related work [12]. These two bright spots corresponded to the (0 0 2) plane of graphite. The d-spacing of the (0 0 2) plane in this carbon shell was determined to be 0.362 nm, slightly larger than the standard value of 0.335 nm for a perfect graphite structure [13]. No extra diffraction spots were observed in the SAED pattern obtained from region A (result not shown). This indicated that the core region was amorphous. The dark field image (DFI) generated by one of the (0 0 2) spots (Fig. 1e) further supported that the silica NW core was engulfed by a graphitic shell structure (bright region in the DFI). The structure of the carbon shell was further examined by HRTEM. The d-spacing of the carbon layer was measured to be about 0.361 nm (Fig. 1f). This result was consistent with the calculated value based on the SAED pattern of the carbon shell. The effects of the reaction temperature on the morphology and structure of the C-SiO2 core–shell nanostuctures was investigated. When the reaction temperature was below 1100 ◦ C, only particles were found on the Si substrate, and no NW was observed. When the system was prepared at 1236 ◦ C for 30 min, the core–shell NWs were obtained. The TEM micrographs (Fig. 2) showed that the carbon shell was thicker than those prepared at 1185 ◦ C (Fig. 1). This suggested that high temperature annealing produced thicker carbon shell on these NWs. The corresponding EDS spectra (Fig. 2b) indicated that the atomic ratio of C:O:Si was 41:19:40 (curve A ) in region A , whereas it was 85:8:7 (curve B ) in region B .
Fig. 3. Raman spectra after the normalization of the samples prepared at (a) 1138 ◦ C for 30 min; (b) 1236 ◦ C for 5 min; and (c) 1236 ◦ C for 30 min.
It was interesting to observe that the SAED pattern (Fig. 2c) obtained from region B of the 1236 ◦ C annealed sample had no diffraction spots. This suggested that its carbon shell was amorphous. The amorphous nature of this carbon shell was further confirmed by the Raman spectra shown in Fig. 3. The wide bands at around 1360 and 1590 cm−1 corresponded to the typical D and G bands of carbon, respectively. The G-band referred to the vibration of the sp2 -bonded carbon atoms in the graphite layer. The D-band was associated with the vibration of carbon atoms with dangling bonds between amorphous carbon. The large value of the intensity ratio between D and G, I(D)/I(G) indicated the randomness of the carbon structure. Comparing curve (a) (sample prepared at 1138 ◦ C
Fig. 2. (a) TEM of one end of a nanowire obtained from the sample prepared at 1236 ◦ C for 30 min; (b) EDS spectra obtained from region A and region B ; and (c) the SAED pattern collected from region B .
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Fig. 4. TEM images of different parts of a NW prepared at 1236 ◦ C for 5 min: (a) the larger end; (b) the smaller end; (c) the EDS collected from the surface; and (d) the HRTEM images of the surfaces arrowed in (b).
for 30 min) and curve (c) (sample prepared at 1236 ◦ C for 30 min), the I(D)/I(G) changed from 0.83 to 1.08. This demonstrated that the carbon shell obtained at high temperature was more amorphous. It was probable that the pyrolysis property of ethanol was responsible to this formation of the amorphous carbon. At high temperature, ethanol was directly pyrolysized into amorphous carbon skipping the catalytic process of forming the graphite layers. A similar situation was reported by Husnu et al. [14]. The effects of reaction time on the carbon shell were also investigated. The micrographs in Fig. 4 showed the different parts of a NW annealed at 1236 ◦ C for 5 min. Again, the corresponding EDS (Fig. 4c) indicated the existence of carbon in the shell of the NW. The typical HRTEM image in Fig. 4d showed that the shell was composed of graphitic sheets in a disorderly manner near the two ends of the silica NW. The d-spacing of graphite layers at the ends were about 0.369 nm and 0.358 nm (Fig. 4d), respectively. The carbon shell in Fig. 4b is about 90 nm in thickness. Comparing with the thickness of the carbon shell in Fig. 2a, we found that the thickness of the carbon shell had been increased from 90 nm (Fig. 4b) to 460 nm (Fig. 2a) when the reaction time was increased from 5 min to 30 min. Recalling the Raman curve b (Fig. 3) for sample prepared at 1236 ◦ C for 5 min, the I(D)/I(G) value was determined to be about 0.81, which was close to curve a but lower than curve c. In other words, the carbon shell formed after 5 min of heat treatment had a graphitic structure, while a longer annealing led to more amorphous carbon. It was possible that the growth of the crystalline shell during annealing had suffered from the lack of catalytic agent. Graphitic sheets could be formed in the initial stage, thereafter, the Au catalyst was surrounded with graphitic sheets thus became ineffective. As a result, only amorphous carbon was directly formed onto the surface of the NWs after the pyrolysis of ethanol during annealing in the later stage of the growth. In contrast to the uniform graphitic carbon shell in the samples prepared at 1183 ◦ C, the carbon shell in the samples prepared at 1236 ◦ C was uneven. Shown in Fig. 4b, the carbon shell was complete. However, the carbon did not form a shell around the big end of the same NW (see Fig. 4a). The same phenomenon was also observed in the samples prepared at 1236 ◦ C for 30 min. The uneven thickness of the amorphous carbon shell was related to the growth mechanism of the structure. The growth of the NWs was expected to grow from the base adhered to the silicon wafer. The end of the structure near the substrate had a younger age than the upper end which was far away from the substrate. Thus, the upper end of the silica NW had a longer time to deposit amorphous carbon and a thicker carbon shell than the end of the NW near the surface of the substrate. Another reason contributed to the uneven carbon shell might be the concentration difference of the carbon clusters in the space near and above the surface of the substrate. As the amorphous carbon came from the decomposition of ethanol, the concentration of the carbon clusters was higher in the space above the substrate
than that near the surface of the substrate. Thus, the upper end of the silica NW had a thicker carbon shell than that on the NW near the surface of the substrate. The possible reactions during annealing could be described by the following equations [15,16]: Si(s) + O2 (g) → SiO2 (s)
(1)
C2 H5 OH(g) → C2 H4 (g) + H2 O(g)
(2)
Si(s) + 2H2 O(g) → SiO2 (s) + 2H2 (g)
(3)
Eq. (1) described the oxidation of silicon by oxygen. The oxygen came from the carrier gas Ar (less than 10 ppm, but enough for the oxidation of silicon to occur as reported in [17]). It has been reported that silica NWs could be produced from a silicon wafer by a simple thermal oxidation without the use of catalyst at high temperature (1300 ◦ C) [17]. The presence of catalyst (such as Au) on the silicon wafer could lower the temperature (850 ◦ C) for the growth of SiO2 nanostructures under a pressure of 10−2 Torr as demonstrated by Liu et al. [18]. In our work, we also found that the SiO2 nanowires could be produced by annealing Au-coated silicon wafer in Ar at 1236 ◦ C. Eq. (2) described the decomposition of ethanol into ethylene and water. The Gibbs free energy of Eq. (2) at 1138 ◦ C was calculated to be −341 KJ mol−1 which suggested that the reaction could be spontaneous. The water produced by the decomposition might also convert the silicon of the wafer to silica dioxide layer as shown in Eq. (3). Graphitic carbon could be obtained from C2 H4 in the presence of the Au catalyst, while amorphous carbon was formed by the directly decomposition of C2 H4 . Herein, the mechanisms in the formation of the graphite coated silica NWs are described as follows. Initially, the Au coating on the Si wafer was transformed into nanosize droplets. At the same time, silicon was oxidized by O2 in the carrier Ar and by the water decomposed from ethanol. The oxidized Si was then dissolved into the Au droplets. Upon saturation, the silica NWs were formed via a solid–liquid–solid route [19]. The graphite shell was deposited from those carbon dissolved into the Au/Si droplet. Under the action of the Au catalyst, the growth was maintained and the shell consisted of carbon sheet with good orientation and uniform thickness. The amorphous carbon shell was produced by the deposition of carbon clusters directly onto the wall of the silica NWs. 4. Conclusions The ethanol CVD method had been designed to fabricate carbon/silica core–shell structure on an Au-coated silicon wafer. In the sample prepared at 1185 ◦ C, the silica NWs were surrounded by graphitic sheets. The growth mechanism of this novel core–shell nanostructure should follow the solid–liquid–solid (SLS) mechanism. However, the carbon shell would become amorphous in the samples prepared at 1236 ◦ C. We proposed that the incoming
J. Zhu et al. / Materials Chemistry and Physics 124 (2010) 88–91
ethanol was pyrolysized into amorphous carbon and these carbon products were directly deposited onto the NWs as amorphous carbon shell. We further observed that the thickness of the carbon shell in sample prepared at low temperature was quite uniform, became uneven on those prepared at higher temperature, and made thicker with increase of reaction duration. Acknowledgements This work was supported by the Hong Kong SAR Government RGC Earmarked Research Grant (Ref. No. 415206; Project code: 2150491) and a Direct Grant from the Faculty of Science, The Chinese University of Hong Kong (Project code: 2060294). References [1] S.V.N.T. Kuchibhatla, A.S. Karakoti, D. Bera, S. Seal, Prog. Mater. Sci. 52 (2007) 699. [2] G.Z. Shen, P.C. Chen, K.M. Ryu, C.W. Zhou, J. Mater. Chem. 19 (2009) 828. [3] Y. Wang, C.Y. To, D.H.L. Ng, Mater. Lett. 60 (2006) 1151. [4] Q. Li, K.W. Kwong, Phys. Rev. Lett. 92 (2004) 1861021.
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[5] A.H. Lu, W.C. Li, N. Matoussevitch, B. Spliethoff, H. Bonnemann, F. Schuth, Chem. Commun. 98 (2005) 98. [6] M. Leonowicz, M. Wozniak, Y.M. Shulga, V.E. Muradyan, Z. Liu, H.A. Davies, W. Kaszuwara, J. Grabski, J. Mag. Mag. Mater. 294 (2005) e57. [7] L. Liao, J.C. Li, D.F. Wang, C. Liu, C.S. Liu, Q. Fu, L.X. Fan, Nanotechnology 16 (2005) 985. [8] K.S. Lee, Y.H. Mo, K.S. Nahm, H.W. Shim, E.K. Suh, J.R. Kim, J.J. Kim, Chem. Phys. Lett. 384 (2004) 215. [9] L.S. Wang, D.B. Buchholz, Y. Li, J. Li, C.Y. Lee, H.T. Chiu, R.P.H. Chang, Appl. Phys. A 87 (2007) 1. [10] X.P. Shen, Z.Y. Jiang, C.L. Gao, Z. Xu, Z.X. Xie, L.S. Zheng, J. Mater. Chem. 17 (2007) 1326. [11] C. Li, F.J. Fang, L.Y. Yuan, N.S. Liu, L. Ai, Q. Xiang, D.S. Zhao, C.X. Pan, X.Z. Zhao, Nanotechnology 18 (2007) 1557021. [12] B.K. Gupta, O.N. Srivastava, New Carbon Mater. 23 (2008) 116. [13] O.E. Andersson, L.V. Prasad, H. Sato, T. Enoki, Y. Hishiyama, Y. Kaburagi, M. Yoshikawa, S. Bandow, Phys. Rev. B 58 (1999) 16387. [14] E.U. Husnu, C. Manish, Nanotechnology 16 (2005) 2153. [15] J. Li, A. Kazakov, F.L. Dryer, J. Phys. Chem. A 108 (2004) 7671. [16] R.C. Jaeger, Introduction to Microelectronic Fabrication, Addison-Weslev, New York, 1988. [17] J.Q. Hu, Y. Jiang, X.M. Meng, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 367 (2003) 339. [18] Z.Q. Liu, S.S. Xie, L.F. Sun, D.S. Tang, W.Y. Zhou, C.Y. Wang, W. Liu, Y.B. Li, X.P. Zou, G. Wang, J. Mater. Res. 16 (2001) 683. [19] S.H. Li, X.F. Zhu, Y.P. Zhao, J. Phys. Chem. B 108 (2004) 17032.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis of carbon coated silica nanowires Jiangtao Zhu, Fung-luen Kwong, Ming Lei, Dickon H.L. Ng ∗ Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, China
a r t i c l e
i n f o
Article history: Received 14 December 2009 Received in revised form 28 April 2010 Accepted 23 May 2010 Keywords: Nanostructures Chemical vapor deposition Electron microscopy Microstructure
a b s t r a c t Carbon coated silica nanowire hybrid structures were directly synthesized by a chemical vapor deposition method using ethanol as the precursor. The morphology and structural properties of the nanostructures were investigated. It was found that, in the sample prepared at a relative lower temperature (1185 ◦ C), the carbon shell was composed of orderly graphitic layers, whereas the carbon shell on those fabricated at higher temperature (1236 ◦ C) was amorphous. Moreover, the thickness of the amorphous carbon shell increased with increasing reaction temperature and reaction time. © 2010 Elsevier B.V. All rights reserved.
1. Introduction One-dimensional (1D) nanostructures such as nanowires, nanotubes and nanobelts have unique properties and promising applications in various fields [1–3]. However, due to their large aspect ratios, the surface features on 1D nanostructures are often found unstable and reactive to surrounding materials [4]. One of the remedies to overcome these problems is to modify the surfaces of these nanostructures. Carbon is one of the most popular materials for this purpose owing to its superior physical and chemical properties [5,6]. Amorphous carbon coated nanowires, in particular, were found to have significant enhanced field emission properties [7–11]. Currently, methods such as DC magnetron sputtering [7], RF induction heating chemical vapor deposition (RFCVD) under hydrogen flow [8], hydrogen arc discharge [9], and the one-step thermal process [10] have been employed to fabricate carbon coated nanostructures. In this work, we have developed a facile route to prepare graphitic carbon coated silica nanowires (NWs) using ethanol as a precursor at ambient pressure. The route had rather high productivity without the necessity of the strict processing condition (e.g. high vacuum) and the usage of hydrogen gas. In the product, a graphitic carbon cylindrical shell was formed around the silica NW core and a typical core–shell structure was produced. The presence of the graphite shell would increase the thermal stability of the silica NW. We had systematically studied the effects of different growing conditions on the thickness and crystalline
∗ Corresponding author at: Department of Physics, The Chinese University of Hong Kong, Room 108, Science Center North Block, Shatin, Hong Kong, China. Tel.: +852 2609 6392; fax: +852 2603 5204. E-mail address: [email protected] (D.H.L. Ng). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.05.071
structures of these silica NWs. A mechanism in the formation of this carbon coated nanostructure is proposed. 2. Experimental Thin layer of Au was coated onto the 1 cm2 Si square substrates (Wafer World, Inc. Test Grade) by ion sputtering a sputter coater (Polaron SC502, Fisons) before they were placed near the center of a horizontal tube furnace. The system was pumped down to 10−2 Torr and was flushed with Argon repeatedly. After returning to ambient pressure and heating up to the temperatures between 1000 ◦ C and 1300 ◦ C, Ar gas was allowed to enter the system at a rate of 40 ml min−1 after passing through an ethanol (99.9%, Merck) bath at 50 ◦ C. The reaction lasted for 30 min, and the samples were collected. A thermal profile of the furnace was measured to determine the exact temperature of each substrate during annealing. The morphologies of the products on the substrates were examined by scanning electron microscopy (SEM, LEO 1450VP). The structures of the products were analyzed by transmission electron microscopy (TEM, Philips CM120) and high resolution TEM (HRTEM, Tecnai 20ST). These microscopes were equipped with X-ray energy dispersive spectroscopy (EDS). Raman measurement was performed on this product by a micro-Raman spectroscope (Renishaw U1000B) utilizing a 514.5 nm solid-state laser as excitation source.
3. Results and discussion On the surface of the Si substrate prepared at 1185 ◦ C for 30 min, a layer of black substance was observed. The SEM revealed that the surface was covered with nanowires (NWs) having an average diameter of ∼500 nm and length of ∼20 m (Fig. 1a). The layer was scratched, and debris was collected and examined by TEM. It was found that these NWs had a core–shell structure (Fig. 1b). The diameter of the core (region A) varied (from 190 nm to 390 nm) along its long axis, whereas the thickness of the shell (region B) was rather uniform (∼80 nm). The corresponding EDS spectra (Fig. 1c) showed that the core (top curve) contained 37 at% Si, 53 at% O and 10 at% C, while the shell (bottom curve) contained 97 at% C, 2 at% O and
J. Zhu et al. / Materials Chemistry and Physics 124 (2010) 88–91
89
Fig. 1. (a) SEM image of the sample prepared at 1185 ◦ C for 30 min; (b) mid segment of a typical core–shell wire; (c) EDS spectra obtained from the core region A and the shell region B in (b); (d) the SAED pattern of the region B, (e) dark field image of (b); and (f) HRTEM image of the edge of the wire (left was the surface of the graphite shell).
only 1 at% Si. It was evident that the core contained mainly silicon oxide, and was covered with a carbon shell. A selected area electron diffraction (SAED) pattern was obtained from region B (Fig. 1d). The two distinguish elongated spots in the SAED pattern indicated that the shell was composed of layered structure with preferred orientation, which was similar to the graphitic layer structure obtained in other related work [12]. These two bright spots corresponded to the (0 0 2) plane of graphite. The d-spacing of the (0 0 2) plane in this carbon shell was determined to be 0.362 nm, slightly larger than the standard value of 0.335 nm for a perfect graphite structure [13]. No extra diffraction spots were observed in the SAED pattern obtained from region A (result not shown). This indicated that the core region was amorphous. The dark field image (DFI) generated by one of the (0 0 2) spots (Fig. 1e) further supported that the silica NW core was engulfed by a graphitic shell structure (bright region in the DFI). The structure of the carbon shell was further examined by HRTEM. The d-spacing of the carbon layer was measured to be about 0.361 nm (Fig. 1f). This result was consistent with the calculated value based on the SAED pattern of the carbon shell. The effects of the reaction temperature on the morphology and structure of the C-SiO2 core–shell nanostuctures was investigated. When the reaction temperature was below 1100 ◦ C, only particles were found on the Si substrate, and no NW was observed. When the system was prepared at 1236 ◦ C for 30 min, the core–shell NWs were obtained. The TEM micrographs (Fig. 2) showed that the carbon shell was thicker than those prepared at 1185 ◦ C (Fig. 1). This suggested that high temperature annealing produced thicker carbon shell on these NWs. The corresponding EDS spectra (Fig. 2b) indicated that the atomic ratio of C:O:Si was 41:19:40 (curve A ) in region A , whereas it was 85:8:7 (curve B ) in region B .
Fig. 3. Raman spectra after the normalization of the samples prepared at (a) 1138 ◦ C for 30 min; (b) 1236 ◦ C for 5 min; and (c) 1236 ◦ C for 30 min.
It was interesting to observe that the SAED pattern (Fig. 2c) obtained from region B of the 1236 ◦ C annealed sample had no diffraction spots. This suggested that its carbon shell was amorphous. The amorphous nature of this carbon shell was further confirmed by the Raman spectra shown in Fig. 3. The wide bands at around 1360 and 1590 cm−1 corresponded to the typical D and G bands of carbon, respectively. The G-band referred to the vibration of the sp2 -bonded carbon atoms in the graphite layer. The D-band was associated with the vibration of carbon atoms with dangling bonds between amorphous carbon. The large value of the intensity ratio between D and G, I(D)/I(G) indicated the randomness of the carbon structure. Comparing curve (a) (sample prepared at 1138 ◦ C
Fig. 2. (a) TEM of one end of a nanowire obtained from the sample prepared at 1236 ◦ C for 30 min; (b) EDS spectra obtained from region A and region B ; and (c) the SAED pattern collected from region B .
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Fig. 4. TEM images of different parts of a NW prepared at 1236 ◦ C for 5 min: (a) the larger end; (b) the smaller end; (c) the EDS collected from the surface; and (d) the HRTEM images of the surfaces arrowed in (b).
for 30 min) and curve (c) (sample prepared at 1236 ◦ C for 30 min), the I(D)/I(G) changed from 0.83 to 1.08. This demonstrated that the carbon shell obtained at high temperature was more amorphous. It was probable that the pyrolysis property of ethanol was responsible to this formation of the amorphous carbon. At high temperature, ethanol was directly pyrolysized into amorphous carbon skipping the catalytic process of forming the graphite layers. A similar situation was reported by Husnu et al. [14]. The effects of reaction time on the carbon shell were also investigated. The micrographs in Fig. 4 showed the different parts of a NW annealed at 1236 ◦ C for 5 min. Again, the corresponding EDS (Fig. 4c) indicated the existence of carbon in the shell of the NW. The typical HRTEM image in Fig. 4d showed that the shell was composed of graphitic sheets in a disorderly manner near the two ends of the silica NW. The d-spacing of graphite layers at the ends were about 0.369 nm and 0.358 nm (Fig. 4d), respectively. The carbon shell in Fig. 4b is about 90 nm in thickness. Comparing with the thickness of the carbon shell in Fig. 2a, we found that the thickness of the carbon shell had been increased from 90 nm (Fig. 4b) to 460 nm (Fig. 2a) when the reaction time was increased from 5 min to 30 min. Recalling the Raman curve b (Fig. 3) for sample prepared at 1236 ◦ C for 5 min, the I(D)/I(G) value was determined to be about 0.81, which was close to curve a but lower than curve c. In other words, the carbon shell formed after 5 min of heat treatment had a graphitic structure, while a longer annealing led to more amorphous carbon. It was possible that the growth of the crystalline shell during annealing had suffered from the lack of catalytic agent. Graphitic sheets could be formed in the initial stage, thereafter, the Au catalyst was surrounded with graphitic sheets thus became ineffective. As a result, only amorphous carbon was directly formed onto the surface of the NWs after the pyrolysis of ethanol during annealing in the later stage of the growth. In contrast to the uniform graphitic carbon shell in the samples prepared at 1183 ◦ C, the carbon shell in the samples prepared at 1236 ◦ C was uneven. Shown in Fig. 4b, the carbon shell was complete. However, the carbon did not form a shell around the big end of the same NW (see Fig. 4a). The same phenomenon was also observed in the samples prepared at 1236 ◦ C for 30 min. The uneven thickness of the amorphous carbon shell was related to the growth mechanism of the structure. The growth of the NWs was expected to grow from the base adhered to the silicon wafer. The end of the structure near the substrate had a younger age than the upper end which was far away from the substrate. Thus, the upper end of the silica NW had a longer time to deposit amorphous carbon and a thicker carbon shell than the end of the NW near the surface of the substrate. Another reason contributed to the uneven carbon shell might be the concentration difference of the carbon clusters in the space near and above the surface of the substrate. As the amorphous carbon came from the decomposition of ethanol, the concentration of the carbon clusters was higher in the space above the substrate
than that near the surface of the substrate. Thus, the upper end of the silica NW had a thicker carbon shell than that on the NW near the surface of the substrate. The possible reactions during annealing could be described by the following equations [15,16]: Si(s) + O2 (g) → SiO2 (s)
(1)
C2 H5 OH(g) → C2 H4 (g) + H2 O(g)
(2)
Si(s) + 2H2 O(g) → SiO2 (s) + 2H2 (g)
(3)
Eq. (1) described the oxidation of silicon by oxygen. The oxygen came from the carrier gas Ar (less than 10 ppm, but enough for the oxidation of silicon to occur as reported in [17]). It has been reported that silica NWs could be produced from a silicon wafer by a simple thermal oxidation without the use of catalyst at high temperature (1300 ◦ C) [17]. The presence of catalyst (such as Au) on the silicon wafer could lower the temperature (850 ◦ C) for the growth of SiO2 nanostructures under a pressure of 10−2 Torr as demonstrated by Liu et al. [18]. In our work, we also found that the SiO2 nanowires could be produced by annealing Au-coated silicon wafer in Ar at 1236 ◦ C. Eq. (2) described the decomposition of ethanol into ethylene and water. The Gibbs free energy of Eq. (2) at 1138 ◦ C was calculated to be −341 KJ mol−1 which suggested that the reaction could be spontaneous. The water produced by the decomposition might also convert the silicon of the wafer to silica dioxide layer as shown in Eq. (3). Graphitic carbon could be obtained from C2 H4 in the presence of the Au catalyst, while amorphous carbon was formed by the directly decomposition of C2 H4 . Herein, the mechanisms in the formation of the graphite coated silica NWs are described as follows. Initially, the Au coating on the Si wafer was transformed into nanosize droplets. At the same time, silicon was oxidized by O2 in the carrier Ar and by the water decomposed from ethanol. The oxidized Si was then dissolved into the Au droplets. Upon saturation, the silica NWs were formed via a solid–liquid–solid route [19]. The graphite shell was deposited from those carbon dissolved into the Au/Si droplet. Under the action of the Au catalyst, the growth was maintained and the shell consisted of carbon sheet with good orientation and uniform thickness. The amorphous carbon shell was produced by the deposition of carbon clusters directly onto the wall of the silica NWs. 4. Conclusions The ethanol CVD method had been designed to fabricate carbon/silica core–shell structure on an Au-coated silicon wafer. In the sample prepared at 1185 ◦ C, the silica NWs were surrounded by graphitic sheets. The growth mechanism of this novel core–shell nanostructure should follow the solid–liquid–solid (SLS) mechanism. However, the carbon shell would become amorphous in the samples prepared at 1236 ◦ C. We proposed that the incoming
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ethanol was pyrolysized into amorphous carbon and these carbon products were directly deposited onto the NWs as amorphous carbon shell. We further observed that the thickness of the carbon shell in sample prepared at low temperature was quite uniform, became uneven on those prepared at higher temperature, and made thicker with increase of reaction duration. Acknowledgements This work was supported by the Hong Kong SAR Government RGC Earmarked Research Grant (Ref. No. 415206; Project code: 2150491) and a Direct Grant from the Faculty of Science, The Chinese University of Hong Kong (Project code: 2060294). References [1] S.V.N.T. Kuchibhatla, A.S. Karakoti, D. Bera, S. Seal, Prog. Mater. Sci. 52 (2007) 699. [2] G.Z. Shen, P.C. Chen, K.M. Ryu, C.W. Zhou, J. Mater. Chem. 19 (2009) 828. [3] Y. Wang, C.Y. To, D.H.L. Ng, Mater. Lett. 60 (2006) 1151. [4] Q. Li, K.W. Kwong, Phys. Rev. Lett. 92 (2004) 1861021.
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